This invention relates to the art of noise discrimination, and more particularly, to apparatus for reproducing a desired signal without reproducing noise that accompanies it.
In the past, it has been the practice to obtain a pulse analog of the velocity of a rotating shaft by passing a toothed ferromagnetic gear-like structure of a magnetic material adjacent a permanent magnet having a coil therearound. An alternating voltage is, thus, induced in the coil. The frequency of this alternating voltage is then directly proportional to the angular velocity of the rotating shaft. This prior art construction suffers from the two serious disadvantages that can increase or decrease the number of cycles of the coil output and thereby give an erroneous velocity analog. For example, vibration can cause the gear-like structure to move back and forth axially toward away from the permanent magnet. Extraneous alternating signals may, thus, be generated in the coil.
It is conventional to employ a velocity analog device in anti-skid braking systems for motor vehicles. In this case, it will be appreciated that vibration can be a substantial problem. Further, in the case of anti-skid braking systems, the coil employed with the ignition system of an internal combustion engine can induce a voltage in the magnetic pick-up for developing an analog of the velocities of two braked wheels.
From the foregoing, it will be appreciated that both the vibration and the induced voltage problems of the prior art make it impossible to obtain an accurate velocity analog. Prior art which is more accurate is more complex and expensive. See U.S. Pat. No. 3,755,745.